On Thu, 15 Jan 2015 17:31:32 +0000 Mike Crowe <m...@mcrowe.com> wrote:
> glibc's dprintf implementation does not work correctly with /dev/kmsg file > descriptors because glibc treats receiving EBADF and EINVAL from lseek when > trying to determine the current file position as errors. See > https://sourceware.org/bugzilla/show_bug.cgi?id=17830 > > >From what I can tell prior to Kay Sievers printk record commit > e11fea92e13fb91c50bacca799a6131c81929986, calling lseek(fd, 0, SEEK_CUR) > with such a file descriptor would not return an error. > > Prior to Kay's change, Arnd Bergmann's commit > 6038f373a3dc1f1c26496e60b6c40b164716f07e seemed to go to some lengths to > preserve the successful return code rather than returning (the perhaps more > logical) -ESPIPE. > > glibc is happy with either a successful return or -ESPIPE. > > For maximum compatibility it seems that success should be returned but > given Kay's new seek interface perhaps this isn't helpful. > > This patch ensures that such a seek succeeds: > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 02d6b6d..b3ff6f0 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -693,7 +693,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t > offset, int whence) > loff_t ret = 0; > > if (!user) > - return -EBADF; > + return (whence == SEEK_CUR) ? 0 : -EBADF; What's actually going on here? What is the significance of file->private_data==NULL and why does this code treat it as an error? > if (offset) > return -ESPIPE; > > @@ -718,6 +718,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t > offset, int whence) > user->idx = log_next_idx; > user->seq = log_next_seq; > break; > + case SEEK_CUR: > + /* For compatibility with userspace requesting the > + * current file position. */ > + ret = 0; > + break; Can we actually do something useful here? Return some value which can be fed back into SEEK_SET to restore the file position? > default: > ret = -EINVAL; > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
